1. Field of the Invention
The present invention relates generally to memory systems in a personal computer system. More particularly, the present invention relates to the manner and techniques by which memory is accessed by components in a computer system. Still more particularly, the invention relates to an improved technique for arbitrating among multiple pending memory requests.
2. Background of the Invention
Modern computer system generally include a plurality of devices interconnected through a system of buses which are linked by way of one or more hubs, or bridge logic units. For example, a conventional computer system typically contains a central processing unit ("CPU") or processor coupled through bridge logic to main memory. A CPU bus usually couples the CPU to the bridge logic, and a memory bus connects the bridge logic to the memory. The bridge logic typically incorporates a memory controller which receives memory access requests and generates the standard control signals necessary to access the main memory. The bridge logic may also include an interface to a high bandwidth local expansion bus, such as the Peripheral Component Interconnect ("PCI") bus. Examples of devices which link to the local expansion bus include network interface cards, video accelerators, audio cards, SCSI adapters, and telephony cards, to name a few. An example of such a bridge logic is described in U.S. Pat. No. 5,634,073, assigned to Compaq Computer Corporation.
Bridge logic may also support an older-style expansion bus through an additional bus interface to provide compatibility with earlier-version expansion bus adapters. Examples of such expansion buses include the Industry Standard Architectures (ISA) bus, the Extended Industry Standard Architecture ("EISA") bus, and the Microchannel Architecture (MCA) bus. Various devices may be coupled to this second expansion bus including a fax/modem, sound card, keyboard, and other peripherals. The bridge logic can link or interface more than simply the CPU bus, a peripheral bus such as a PCI bus, and the memory bus. In graphics-intensive applications, bridge logic may support a separate peripheral bus optimized for graphics related data transfers. A popular example of such a bus is the Advanced Graphic Port ("AGP") bus.
Because many of the devices interconnected through this series of buses function independently of each other, they often attempt to access common resources concurrently. For example, a device coupled to the AGP bus may need to extract data from main memory to drive a video display at the same time the CPU is requesting instructions stored in main memory that allow the video program to run. Both actions require memory access, and the memory controller must choose which device (the CPU or the AGP device in this example) to service first. Such conflicts necessitate arbitration, in which priority ratings are assigned to rank memory requests and allow the memory controller to service memory requests by order of importance.
Since computer systems have traditionally been developed for business applications including word processing and spreadsheets, among others, the arbitration schemes within such systems have generally been geared to guarantee the highest memory access priority to a single device (such as the CPU), with lower priority rankings assigned to the remaining devices in the computer. Using such a fixed priority scheme, a memory controller, if faced with multiple pending memory access requests, simply grants memory access to the device with the highest priority, traditionally the CPU. A fixed-priority arbitration scheme is generally well-suited to the more traditional computer application programs, which tend to involve relatively minor levels of user input, device interaction, and graphics output.
Recently, however, computer systems have been increasingly employed in processing real-time data, including multimedia applications such as video and audio, telephony, and speech recognition. The performance of these applications suffers if the computer cannot process the real-time data within a minimum time period. When the CPU or other highly prioritized device issues numerous memory access requests, the memory controller is prevented from granting access to lower-ranking devices, even if those devices are processing real-time data. Until the high-priority devices stop issuing memory access requests, the low-priority real-time applications are forced to stall and wait for access to memory. Accordingly, fixed-priority memory arbitration techniques may be unacceptable for real-time applications, such as video, where unintended pauses in the on-screen action can ruin the effect of a moving picture, or in speech recognition, where failure to capture portions of the speech data can prevent the computer from recognizing what a speaker said. Hence, fixed memory arbitration schemes often are inadequate to support the memory demands of emerging computer applications.
The Least-Recently-Used (LRU) algorithm, in which a memory arbiter grants the request which has least recently been granted, is one alternative to fixed arbitration schemes since the priority structure of an LRU scheme may change in response to the memory request sequence. However, this type of responsive priority change essentially equalizes, or fixes, the priority of all devices in the computer system, since the arbitration scheme does not take into account the urgency associated with memory transactions from certain devices. Further, the devices which use memory infrequently actually tend to experience shorter waits for memory access, since these devices are less likely to have recently accessed memory than are devices which access memory more frequently. As a consequence, real-time applications and devices, which need frequent and quick access to memory, may consistently lose memory arbitration to other devices under an LRU scheme. Hence, an LRU scheme, while more equitable that a fixed scheme, lacks the flexibility to allow the computer system designer to directly set the memory request priorities.
For the foregoing reasons, it would be advantageous to design a computer system that includes a bus bridge architecture that permits all devices in a computer system fair access to memory, without incurring the drawbacks of current arbitration methods. A memory controller with an equitable, yet configurable, arbitration scheme could dramatically improve the quality of service associated with memory accesses in modern computer applications. Despite the apparent advantages that such a system would provide, to date no such system has been developed that provides these features.